I also have 2x Zotac GTX 680 4GB in SLI. Im getting quite good performance in BF3, around minimum of 45fps and avg of 65fps.
Altough i think it could do better and im wondering if a third GTX 680 4GB would be worth it. like, could i turn on MSAA and still get a smooth experience?.

As for BF3 im playing with all ultra and AA post on high with a FOV of 90.